Field of the Invention
The separation of materials can be accomplished by the application or the influencing of forces on intrinsic species of the material or on species of a compound of materials. These forces can be gravitational, electromagnetic, chemical, gas (fluid) dynamic and/or a combination of these forces. The forces will provide different acceleration to the species thereby giving different spatial and temporal characteristics to the desired separation. The material could be in the solid, liquid, vapor or plasma state, or a combination of these states. The magnetoplasmadynamic process encompasses each or a combination of these states.
The magnetoplasmadynamic phenomena was first discovered in 1961 by Cann and described in U.S. Pat. No. 3,243,954. A device that was designed and tested was intended for space propulsion applications and is commonly known as an ion propulsion system. The principal design and performance requirements were to fully ionize a single species vapor and accelerate all the ions to a preferred high velocity into space. The device was to have high thrust efficiency, where thrustor efficiency was defined as EQU Thrust efficiency=.eta..sub.mass .multidot..eta..sub.exhaust .multidot..eta..sub.power
where ##EQU1## where EQU m.sub.ions =mass flow of ions EQU .nu.=average velocity of ions
No attempt was made to separate materials in this process as the intended application did not direct the development to that end.
The magnetoplasmadynamic phenomenon simply comprises the controlled interaction of a plasma and an applied magnetic field through the induced magnetic field resulting when the plasma is accelerated by an appied electrostatic field (potential). This type of interaction phenomena is referred to as the Hall Current effect. The significance of the work of Cann and others was the controlled feed or propellant injection into the cavity anode region. The proper voltage selection and propellant injection rate resulted in ionization and acceleration of the charged particles (ions and electrons) in the direction parallel to the applied magnetic field. The resulting plasma was accelerated to the desired exhaust velocity.
This invention relates to the separation of materials by new and improved methods employing magnetoplasmadynamics. The invention allows the separation and collection of materials that cannot be separated by other techniques or processes. The invention also separates materials that can be separated by other techniques or processes but this invention would separate the material at lower costs.
An important use of methods and apparatus of this invention is the production of semiconductor grade silicon from low-cost silicon compounds as the feed material. Semiconductor grade silicon is considered to be silicon which is made at least 99.9999% pure by refining techniques. For efficient operation of silicon solar cells the semiconductor grade silicon should actually be at least 99.999% pure and preferably 99.9999% pure. On the other hand, silicon which is at least 97% pure is considered to be metallurgical grade silicon. Commercially available metallurgical grade silicon is approximately 98% pure. The cost of semiconductor grade silicon is significantly greater than that of metallurgical grade silicon. In 1980, semiconductor grade silicon cost approximately $80./Kg, whereas metallurgical grade silicon cost less than $1.00/Kg.
Ironically, the semiconductor grade is valued because it is usually used in products which require that specified impurities or "dopants" be added to the silicon. These dopants affect conductivity of the silicon and create donor and receptor portions on semiconductor devices. Therefore, as used in this specification, "semiconductor grade silicon" should be construed to include the highly pure silicon which has dopants added.
As noted, the semiconductor grade silicon is used to produce divers semiconductor devices, including silicon solar cells. It is essential that the cost of solar cell production be reduced so that the cost of production of solar cells be recovered over the expected lifetime of the solar cells. At the present time silicon solar cells are made from single crystals of n-type silicon about 4 centimeters in diameter and as long as a sausage. These crystals are made by rotating and pulling at a slow regulated pace. The elongated crystal is then cut into slices approximately 50 microns thick by means of a diamond-tipped circular saw. After the slices are ground, lapped and chemically cleaned, they are placed in a diffusion chamber which consists of a long quartz tube running through a cylindrical electric furnace. In the diffusion chamber, the crystals are heated to 1150.degree. in an atmosphere of boron trichloride. Elemental boron, which decomposes from the boron compound, diffuses into the outer surfaces of the silicon wafers, thus doping the wafers to create a p-type layer less than 0.3 microns thick. Further processing is necessary to create the terminals and to expose the n-type silicon, now sandwiched in the middle.
While this process may be ideal for the production of small electronic components, the steps involved put the price of solar cell panels at over $12,000.00 (1973 dollars) per kilowatt of generated power. While the real cost of production is expected to decrese in accordance with the refinement of production techniques and mass production, the price is still several orders of magnitude too large for solar panels to be used commercially for power production in other than remote areas such as space ships. Also, in many cases the energy spent in production cannot be recovered over the expected lifetime of the solar cells.
Several systems have been proposed for the production of semiconductor devices, such as solar cells, by other means. For example, Janowiecki, et al., U.S. Pat. No. 4,003,770 (also U.S. Patent Office Voluntary Protest Program Document No. B 65,105) discloses a process for preparing solar cells in which p- or n-doped silicon particles are injected into a plasma stream where the particles are vaporized. The heated particles are then discharged from the plasma stream onto a substrate to provide a polycrystalline silicon film. During the heating and spraying, a suitable atmosphere is provided so that the particles are surrounded to inhibit oxidation. However, Janowiecki, et al. do not suggest the use of his techniques for refining the silicon. Walter H. Brattain in U.S. Pat. No. 2,537,255, discloses the deposition of silicon for silicon photo-emf cells, using a mixture of hydrogen and silicon tetrachloride. However, this early technique does not disclose the use of a magnetoplasmadynamic effect for either production of these solar cells, nor the refining of metallurgical grade silicon into semiconductor grade silicon.
Tsuchimoto in U.S. Pat. No. 3,916,034 discloses a method for transporting semiconductors in a plasma stream onto a substrate. The plasma is directed by magnetic fields onto thin film substrates. Tsuchimoto is typical of conventional ionization chambers for use with a magnetogasdynamic process. In that magnetogasdynamic process, mass utilization efficiency is low, making the method ineffective for refining mass quantities of silicon.
It is also possible to provide a deposition system using a magnetoplasmadynamic arc in which a magnetic nozzle is produced by a magnetic coil and/or the self-magnetic field of discharge. This magnetic nozzle permits a higher ion density in the plasma jet, giving better control over the distribution of silicon in the jet. The anode attachment can be diffused or made to rotate rapidly during the electromagnetic (j.times.B) forces, permitting uniform erosion of the anode. However, this system provides a "modal" performance, in that small changes in the mass flow rate critically affect the voltage requirements of the system. In this system, an insulator between the anode and cathode is subject to erosion or may short out the discharge by being coated during the operation of the plasma device.